1. Field of Application
Coal combustion fly ash has been marketed for a variety of applications. The addition of fly ash to cement is an expanding industry and accounts for a huge opportunity for the coal burning power plants to defray operating costs for disposal. However if the ash does not meet certain guidelines in terms of residual carbon; or has surfactant adsorption problems-measured in terms of a foam index; or contains ammonia, the fly ash is not only unusable but becomes a disposal problem. This application is concerned with ways and means to treat unmarketable fly ash and improve its properties, use and value. It is of great importance to the power, cement and construction industries.
2. Description of Known Art
Fly ash derived from power plants is frequently used in the production of concrete. According to the relevant ASTM requirements, fly ash may replace cement in concrete up to 5-10%.
Generally speaking if the carbon in the fly ash were less than 3% w/w (usually measured as loss in ignition or LOI, which closely correlates with the carbon content) it would be marketable. This is common with the C class ashes, which are generally from sub-bituminous coals. Quite often the bituminous coals, which lead to F class ashes, have much higher carbon levels and may in any event be unmarketable unless the carbon is burned out or removed by a separation process.
In the preparation of concrete known as “ready mix” air-entraining agents are used to allow the introduction of micro bubbles into the concrete. These micro bubbles aid in the control of expansion and contraction of the concrete as occurs with freeze and thaw in the environment. Too much air entrained in the concrete reduces strength and too little results in poor adaptation to the above weathering impacts. Many of the ashes can have a seemingly low carbon or LOI level and yet have a problem with the quantity of surfactant required to entrain micro bubbles. It has been determined that it is the carbonaceous residual in fly ash impacts the adsorption of surfactants. This carbon has a high surface area and is very active towards the so-called air entraining agents.
The normal cause of the problem with the fly ash is: low and ultra low NOx burner systems starve the combustion of the coal in the primary firing zone and then add over-fire air to complete oxidation. While minimizing NOx formation these conditions can create higher levels of condensed organic carbonaceous intermediates or soot in or on the fly ash, as well as incompletely combusted particles of (devolatilized) coal. They are effectively incomplete combustion products of the carbon. The remaining mineral matter then has a significant quantity of both coarse (−50 microns) and nano-particulate carbon intermingled with it. This material has a tendency to adsorb the surfactants or air entraining agents causing a high “foam index” and it is thought that it is the finer particles that create most of the problem. This high foam index relates to the number of drops of a standard surfactant that is added to a known quantity of fly ash under controlled conditions. A high foam index material requires much more surfactant than normally necessary.
It is not often clear whether the cause of the high foam index is the reducing conditions in the combustion zone, the residence time of solids in the gas phase, or the load on the boiler unit. It is most probably a function of the specific surface area per unit volume of the carbon, which is related inversely to the particle size (diameter). These effects are not fully understood. Suffice it to say, there are many plants that have this problem with their fly ash. They are struggling to find a solution to overcome the issue so the ash may be sold rather than disposed. Additionally, the situation is worsened by the lack of knowledge of the correlation with the potential causes and the variability of the foam index value. It will be clear to the reader that the variable nature of the foam index will lead to an unknown or un-quantifiable amount of air entraining agent—one minute the amount added if constant might be too much and the next too little.
Another problem with fly ash arises from the presence of ammonia. If too high a concentration is present in the fly ash it may release upon addition of water and the other cement components leading to deleterious environmental consequences. Emission of ammonia is not only unpleasant in closed-in working environments; it is also toxic in high enough concentrations.
Ammonia in fly ash from power plants is created by the injection of ammonia to remove NOx. This technique is employed with Selective Catalytic Reduction (SCR) units. These control devices employ special catalysts, which combine ammonia with NOx components and form harmless nitrogen and water. Some stoichiometric excess is needed to assure the removal of the NOx, but over-injection creates ammonia “slip”, as it is referred to, which results in high local concentrations of the undesirable component adsorbed onto the particulates (or fly ash) in the gas stream. A material so affected is difficult to sell into the concrete market.
These two problems occur in fly ash resulting from coal burning power plants where there are emission control features installed on the boilers. There are innumerable cases where there is need for correction of the either of the two issues and sometimes both on the same plant.
Thermal oxidation is a process that can remove both the ammonia and the carbon foam index issue. Indeed, carbon burn out facilities have been constructed that will take the carbon content and burn it to a low level and it is known that the foam index problem is destroyed under these circumstances. Temperatures for such processes are generally in the region of 700-850° C. Ammonia is also broken down by thermal oxidation means and this can take place at lower temperatures around 350 to 500° C.
Where the foam index issue has to be addressed without real burnout of the carbon (usually because there is insufficient to provide combustible heat release of any magnitude) the foam index can be ameliorated or lessened by thermal treatment at temperatures as low as 400° C. but generally more like 700° C. Naturally the requirement to heat up the fly ash to any of these temperatures results in the expenditure of quite a lot of energy or fuel. Some of this can of course be recovered in the gas stream to preheat the feed air for the combustion process, but nevertheless the demand for fuel is quite significant.
For the treatment of foam index problems, Hurt et al U.S. Pat. Nos. 6,136,089 and 6,521,037 have proposed and patented the use of ozone. This has the advantage of being applied at low temperatures negating the fuel requirements of a truly thermal or combustion process. The dosage rates are relatively high, though, and the use of ozone has an energy demand of its own. First, the production of ozone is inefficient due to side reactions and the overall power required for a reasonable concentration is quite significant. Secondly, ozone is made from an air stream or an oxygen stream or a mixture of both—but the gas must be almost completely dry (free of water). The generally known conditions for its production are a low dew point of −40° C. or lower in the gas stream, as it is unstable in moist air. Hence, an important part of the power consumed in its generation comes from the drying of the air stream to low dew points. Alternatively, oxygen may be used (which has an inherently low dew point due to the manufacturing processes—from cryogenics or pressure swing methods both of which eliminate water from the gas as an early step in the process chain). However, the power for production of oxygen has to be taken into account—this can range from about 200-400 kWh/ton. Oxygen enables higher concentrations of ozone to be made. Ozone made from oxygen can, for example, be as high as 1-6% w/w. Whereas the concentration level is significantly lower with air perhaps 200 ppm to 5,000 or 10,000 ppm.
The reaction, which produces ozone from a corona discharge in air or oxygen, is quite endothermic. The inefficiencies in the process lead to the release of energy as heat. This heat must be removed from the gas during production. If this heat is not removed, by a cooling circuit, the ozone breaks down—and effectively the product concentration is lowered significantly. The cost associated with the cooling duty is an additional power load and is often not figured in the production cost of ozone generators. This is the third source of cost.
Lastly, the capital cost of ozone generators is significant, as they require close tolerance in manufacture for fitting the dielectric inserts and electrodes within the tubular arrays in a concentric manner. This impacts the overall operating cost in terms of depreciation charges.
The power associated with production is in the range of 9 to 18 kWh/kg of ozone: this, coupled with the relatively high dosage rate required for fly ash treatment, can make this quite an expensive proposition.
Hurt et al identified the conditions for use of the gas to oxidize the carbonaceous material on some fly ash materials tested. To achieve the desired effect for foam index reduction, dosage ranges from 0.5 to 2 or 3 g of ozone/kg of fly ash. Ozone is toxic and needs to be utilized fully or broken down into atmospheric oxygen after the atomic oxygen has taken part in the reaction step. This in itself requires very careful management of the contacting and dosage rate or a back up catalytic breakdown system using manganese dioxide or thermal treatment to about 300° C. At the latter temperature, the residual ozone is reduced to negligible levels.
The authors also elaborated on the mechanism and noted that the actual LOI figure increased with the ozone application. This implies a different mechanism from a breakdown of carbonaceous material into components such as carbon dioxide and water vapor unless they are still held as by-products on the surface.
In international patent WO 02/097330 A1J.M, Tranquilla discloses the use of a microwave reactor together with a carbon-free material and oxygen contacting for reduction for the carbon content in high carbon fly ash. However, the operating temperatures employed with the technique are above 600° C., which leads to the expenditure of significant microwave energy for its attainment. This is, therefore, a variant on a high temperature process for burnout of carbon in fly ash. While this process reduces the carbon content significantly, it is not specifically targeted at lowering the foam index or ammonia removal. The low temperature and energy requirement is the stated objective of this present application.
Another form of energy that has been applied to accelerate reactions and, in particular, oxidations in the field of organic chemistry and wastewater treatment is ultraviolet radiation. However, this has insufficient energy, at close to ambient temperatures, to engage in burnout processes. It also requires relatively accessible surfaces, high surface area, thin layers of material, if the material is solid, or rapid material exchange within the body of the material to be effective.
Other inventors have sought to utilize chemical injection or spray treatment to modify the surface of the fly ash and passivate the fly ash surfactant demand. While this is a low temperature application, which minimizes the energy and is applied to dry fly ash, it has possible future unpredictable consequences for the concrete. Dosing and application rate are a practical issue, which are difficult to control with material that is being transferred at high rate into tankers for dispatch. Generally there is poor penetration into the heart of the flowing mass. The chemicals themselves are potentially hazardous if spilled in transit and are often aliphatic or aromatic carboxylic acids and their salts—see U.S. Pat. No. 6,599,358.